There are many different signal processing applications that benefit from adjusting the relative behavior of certain signal components. This so-called equalization can boost or weaken the energy of a specific frequency or frequency band, or adjust other frequency-dependent signal attributes such as phase or time delay. A time-interleaved analog-to-digital converter (TIADC) is one such application. A TIADC uses N parallel channels, each of which runs at a sample rate that is 1/N of the system sample rate, to convert an analog waveform into a digital signal. Each channel has its own sample-and-hold (S/H) circuit coupled to a respective ADC, which trigger on every Nth clock pulse from the TIADC's clock. In a four-channel TIADC, for example, the first channel samples the analog input on the first, fifth, ninth, etc. clock pulses; the second channel samples the analog input on the second, sixth, tenth, etc. clock pulses; and so on. A digital multiplexer interleaves the digital outputs from each channel to produce a fullband digital representation of the analog input waveform.
TIADCs can operate at higher frequencies with greater dynamic range than conventional ADCs, making them suitable for software-defined radio, test and measurement (e.g., in oscilloscopes), medical imaging, precise medicine dispensers (fluid flow measurement), synthetic aperture radar, digital beam-forming communication systems, and other applications that require high-speed analog-to-digital conversion.
Ideally, the timing phases of the sampling clocks to the constituent converters are adjusted such that the digitized samples at the output of the TIADC are spaced evenly in time. This enables a TIADC to achieve an effective sample rate of N times the sample rate of any individual converter operating in isolation, where N is the number of on-chip ADCs. However, small gain and phase mismatches in the linear and nonlinear responses of the individual ADCs create unwanted spurs that can in some cases dominate the TIADC's spurious and/or intermodulation-free dynamic range. These distortions, referred to here as “mismatch distortions,” are nonlinear distortions that occur at frequencies that do not correspond to polynomial combinations of the input signal. Spurious mismatch distortions may make the TIADC output unsuitable for further processing.